J. V. Galan

Ultrafast All-Optical Switching in a Silicon-Nanocrystal-Based Silicon Slot Waveguide at Telecom Wavelengths

We demonstrate experimentally all-optical switching on a silicon chip at telecom wavelengths. The switching device comprises a compact ring resonator formed by horizontal silicon slot waveguides filled with highly nonlinear silicon nanocrystals in silica. When pumping at power levels about 100 mW using 10 ps pulses, more than 50% modulation depth is observed at the switch output. The switch performs about 1 order of magnitude faster than previous approaches on silicon and is fully fabricated using complementary metal oxide semiconductor technologies.

Single Bandpass Photonic Microwave Filter Based on a Notch Ring Resonator

A novel tunable single bandpass photonic microwave filter is proposed. It is based on optically filtering one of the sidebands of a phase-modulated optical carrier by means of the notch response of a silicon-on-insulator ring resonator. The filter response can be tuned by changing the laser wavelength. Experimental results to prove the concept are provided.

Highly efficient crossing structure for silicon-on-insulator waveguides

A compact waveguide crossing structure with low transmission losses and negligible crosstalk is demonstrated for silicon-on-insulator circuits. The crossing structure is based on a mode expander optimized by means of a genetic algorithm leading to transmission losses lower than 0.2 dB and crosstalk and reflection losses below 40 dB in a broad bandwidth of 20 nm. Furthermore, the resulting crossing structure has a footprint of only 6x6 microm(2) and does not require any additional fabrication steps.

Low-Crosstalk in Silicon-On-Insulator Waveguide Crossings With Optimized-Angle

Low crosstalk losses in silicon-on-insulator (SOI) waveguides are demonstrated. The proposed crossing structure has a high compactness, a broad bandwidth with almost flat transmission losses and constant crosstalk losses and is robust against fabrication inaccuracies.

Squeezing and expanding light without reflections via transformation optics.

We study the reflection properties of squeezing devices based on transformation optics. An analytical expression for the angle-dependent reflection coefficient of a generic three-dimensional squeezer is derived. In contrast with previous studies, we find that there exist several conditions that guarantee no reflections so it is possible to build transformation-optics-based reflectionless squeezers. Moreover, it is shown that the design of antireflective coatings for the non-reflectionless case can be reduced to matching the impedance between two dielectrics. We illustrate the potential of these devices by proposing two applications in which a reflectionless squeezer is the key element: an ultra-short perfect coupler for high-index nanophotonic waveguides and a completely flat reflectionless hyperlens. We also apply our theory to the coupling of two metallic waveguides with different cross-section. Finally, we show how the studied devices can be implemented with non-magnetic isotropic materials by using a quasi-conformal mapping technique.

Tailoring the dispersion behavior of silicon nanophotonic slot waveguides

We investigate the chromatic dispersion properties of silicon channel slot waveguides in a broad spectral region centered at ~1.5 μm. The variation of the dispersion profile as a function of the slot fill factor, i.e., the ratio between the slot and waveguide widths, is analyzed. Symmetric as well as asymmetric geometries are considered. In general, two different dispersion regimes are identified. Furthermore, our analysis shows that the zero and/or the peak dispersion wavelengths can be tailored by a careful control of the geometrical waveguide parameters including the cross-sectional area, the slot fill factor, and the slot asymmetry degree.

Study of asymmetric silicon cross-slot waveguides for polarization diversity schemes

We study cross-slot waveguides for polarization diversity schemes that simultaneously offer strong confinement in the slot region for both TE and TM polarizations. A symmetric configuration is initially presented to demonstrate that the same strong confinement and propagation constants can be obtained for both polarizations and slot thicknesses down to 50 nm. To make easier further realization of these waveguides, an asymmetric waveguide configuration with a vertical slot height of up to 120 nm is proposed. The waveguide parameters are then optimized taking into account a 220 nm silicon thickness. Our simulation results show that no beating issues between TE and TM modes is observed for beating lengths under 6.68 microm, thus opening the way to efficient implementations of polarization diversity approaches.

Polarization insensitive low-loss coupling technique between SOI waveguides and high mode field diameter single-mode fibers.

A polarization insensitive technique for highly efficient coupling between SOI waveguides and high mode field diameter single-mode fibers is reported. The proposed coupling structure is based on an inverted taper structure coupled to a fiber-adapted waveguide. The fiber-adapted waveguide is made by using the SiO(2) layer under the Si waveguiding layer of the SOI wafer thus avoiding the use of extra materials such as polymers. The proposed coupling structure is aimed for being integrated with V-groove auto-alignment techniques. Coupling losses of 3.5dB and 3.7dB to 8mum mode field diameter single-mode fibers have been estimated by means of 3D-BPM simulations for TE and TM polarizations respectively and a 1550nm input signal wavelength.

Group velocity dispersion in horizontal slot waveguides filled by Si nanocrystals

A study of group velocity dispersion of horizontal slot waveguides filled by Si nanocrystals with different Silicon concentrations has revealed a change in the sign of GVD from negative to positive values across the third telecom window.

Group-index engineering in silicon corrugated waveguides.

We present experimental measurements of the group index in tailored silicon corrugated waveguides. Nearly constant group index as high as ng=14 ±0.5 in a 13 nm range was measured in a 50 µm long waveguide.